The present invention relates generally to magnetic disk devices having separate heads for writing (recording) and reading (reproducing) data, and more specifically to compensating timing for starting writing to account for different distances between the heads.
FIG. 4 is a diagram showing how the data is typically written and read in a conventional magnetic disk device 1 in which data sectors (minimum addressable units for writing and reading) are arcs formed at specified positions on concentric circles (tracks). Servo signals 11 are written on the magnetic disk 1 in advance and are spaced uniformly in the rotation direction and include information in the radial direction. When the data is written and read in a specified data sector 12, a magnetic head 2 is first positioned with respect to the radial direction of the magnetic disk 1 by moving a rotation system support arm (not shown), to which the magnetic head 2 is attached, based on the information of the servo signals 11.
Next, a commanded position in the rotation direction on the disk is determined with reference to any signal identifying a specific physical position on the disk such as, for example, the end of the servo signals 11. Clocks and the like provide timing signals that are used to establish the waiting time T until the magnetic disk 1 is rotated so that a magnetic head 3 is positioned at the top of one of the data sectors. Typically, there are several data sectors between the servo signals 11. When the magnetic disk has rotated the correct distance, a write gate or a read gate is asserted.
When the write gate is asserted, a write current flows through the magnetic head 3 so that a signal is converted by the magnetic head 3 into magnetic information and written on the magnetic disk 1. On the other hand, when the read gate is asserted, the magnetic information on the magnetic disk 1 is converted into electric information by the magnetic head 3 and, then, the data is sent to a host through circuits such as an amplifier.
As described above, physical positions on the magnetic disk must be determined in order to assert the write gate and the read gate and, then, the signals to determine the physical positions on the magnetic disk should be determined based on the servo signals read from the read head. FIG. 5 shows a deviation in timing between the write head and the read head in the prior art wherein, in a magnetic disk device in which the write head and the read head are provided separately and the data is written and read by both heads as shown in FIG. 5, in the case where the write head 2b is apart from the read head 2a by a distance L in the backward direction, when a duration equivalent to the waiting time T has elapsed from the reference position defined by the servo signal 11, the read head 2a is placed at the top of the data sector 12 but the write head 2b is placed not at the top of the data sector 12 but at the preceding sector. Therefore, if the write gate is asserted to start writing at this timing, the data on the preceding sector would be overwritten. Further, if the data is written on the data sector disposed just after the servo signal, the servo signal may also be overwritten.
When the data is written using such magnetic heads, the write gate must be asserted to write the data after the magnetic disk has rotated a sufficient amount so that the write head is positioned at the top of the desired data sector as the magnetic disk rotates. This time gap TG depends on a head gap distance L of the write head and the read head as well as a relative speed v between the magnetic disk and the magnetic heads. Therefore, in a magnetic disk device in which the magnetic disk rotates with a uniform speed, as the relative speed v between the magnetic disk and the magnetic heads varies according to the radial position on the magnetic disk, the time gap TG must be compensated according to the radial position and a measure against the variation of the relative speed according to the radial position has been proposed (see Japanese Patent Laid-open Hei 6-176486).
Further, as the prior art, a concept in that values of the time gap TG resulted from the head gap distances L that vary from one head to another are measured to compensate the time gap TG for each head has been proposed (see Japanese Patent Laid-open Hei 09-115124).
In this connection, generally speaking, in the magnetic disk devices, as the head gap distance L is a value unique to each head and may vary in some extent due to manufacturing reasons, the time gap TG to be compensated may vary from one head to another. Further, based on the fact that it is difficult to measure the head gap distance L for each head, as the head gap distance L is replaced by a representative value and only the variation due to the relative speed v between the magnetic disk and the magnetic heads is compensated, the head-to-head variation of the time gap TG is accommodated by providing redundancy in the format of the data sectors as shown in (1) of FIG. 6 (or, more specifically, by adding areas A, B and C as shown in the figure, which will be described in detail later). Here, it is to be noted that FIG. 6 is a diagram for describing the format when the variation of the gap between the write head and the read head is not compensated.
Typically, the format of the data sectors includes minimal required areas consisting of: a Preamble area for compensating deviations in frequency and phase between writing and reading; a Sync area for indicating the top of the data; and a Data area including information to be written. However, the format shown in FIG. 6 further includes the extra areas A, B, and C for accommodating the variation of the time gap TG.
The areas A and B are needed so as to prevent forward and backward sectors from being overwritten due to the variation of the time gap TG. As described above, the write gate must be asserted when the write head 2b is positioned at the top of the data sector 12. However, as the physical positions on the magnetic disk can be determined by the servo signals read through the read head 2a, the positional relationship between the read head 2a and the magnetic disk can be known accurately but the determination of the position of the write head 2b should inevitably depend on the setting of the time gap TG.
Therefore, if the write operation is performed with a uniform time gap TG without compensating the head-to-head variation, the write gate will be asserted with uniform timing regardless of the head gap distance. At this time, if the data is written on the data sector 12 having an identical address by the head having a head gap distance L larger than the design value, the data may be written at a physical position that is offset forward from the position where the data ought to be written as shown in (2) of FIG. 6 and, therefore, the data on the receding sector may be overwritten. The area A is necessary to prevent this problem or, in other words, an area where the data is not written must be provided in the forward of the data sector so as to compensate the forward offset of the write position due to the variation of the head gap distance.
Similarly, if the data is written by the head having a head gap distance L smaller than the design value, as the data may be written at a physical position that is offset backward from the position where the data ought to be written as shown in (3) of FIG. 6, in order not to overwrite the data on the subsequent sector, the area B where the data is not written must be provided in the backward of the data so as to compensate the backward offset of the write position due to the variation of the head gap distance.
On the other hand, the area C is needed when the data is read. Typically, the read operation is started by asserting the read gate wherein, first, the deviations in frequency and phase from the data as written are corrected by a PLL circuit. This correction is performed in the Preamble area and, generally speaking, a constant periodic pattern, which is ready to be corrected, is written on the Preamble area. Then, the Sync pattern that indicates the top of the data is searched and, if the Sync pattern is found, the data after the Sync pattern is transferred to the host.
It is possible to know the physical position of the read head 2a on the magnetic disk and, therefore, the read gate can be asserted accurately at the top of the data sector 12 but, as the physical position where the data is written may be offset forward or backward due to the variation of the head gap distance in the write operation as described above, the signal may be read from the data with different timings depending on the variation of the head gap distance. At this time, if the Preamble area, which is necessary to correct the deviations in phase and the like of the timing to assert the read gate by the PLL circuit, cannot be obtained, the deviations in phase cannot be corrected sufficiently and the subsequent Sync area and the data cannot be read accurately and, therefore, the timing to assert the read gate will be synchronized with the head having the largest head gap distance by which the data is written at the most forward position.
However, when the read gate is asserted with such timing, if the head has a smaller head gap distance, the written data is offset backward and, therefore, no signal state will continue for a certain time period after the read gate is asserted. In the situation where the deviations in phase and the like is corrected by the PLL circuit after the read gate is asserted, the operation of the PLL circuit may be unstable and the subsequent correction may be impossible if the no signal state continues while the PLL circuit is operated and, therefore, the signal identical to the one written in the Preamble area must be written over the entire area C.
As there are these redundant areas in the format shown in FIG. 6, a portion that can be used effectively as the data area is reduced or, in other words, the formatting efficiency is reduced and, therefore, the write density must be increased in order to achieve the theoretical data storage capacity of the entire magnetic disk device.
In Japanese Patent Laid-open Hei 6-176486 described above, though the time gap TG corresponding to the head gap distance L between the write head and the read head is compensated according to the radial position on the magnetic disk, it is not contemplated to compensate the head-to-head difference of the head gap distance L.
On the other hand, in Japanese Patent Laid-open Hei 09-115124 described above, a concept in that the head-to-head difference of the head gap distance L is compensated so as to eliminate the redundant areas (the areas A, B and C) described above and increase the formatting efficiency is proposed at least, wherein the time gap TG is measured by employing an unwritable area (a splice) created due to the delay after the write gate is asserted until the write amplifier outputs the write current. However, in this measuring method, the splice may greatly depend on characteristics of the write amplifier and the length and other properties of the splice may vary according to circumstances, it is very difficult to detect the position of the splice.